1. Field of the Invention
The present invention relates to an in-mold foam molding method employing prefoamed beads comprising polyolefin synthetic resin, and to in-mold foam molded articles.
2. Description of the Related Art
Existing in-mold foam molding apparatuses for producing molded articles using prefoamed beads comprising thermoplastic synthetic resins include apparatuses comprising a core mold and a cavity mold; filling means for filling the mold cavity formed between these two molds with prefoamed beads; steam feed means for passing steam through the prefoamed beads packed into the mold cavity in order to heat, foam, and fuse the preformed beads; and cooling means for cooling the molded article by spraying cooling water onto the back faces of the core mold and cavity mold. Chambers are defined to the rear of the core mold and of the cavity mold, and the core mold and the cavity mold are provided with air orifices, such as core vents or core vent holes, which communicate with the mold cavity. During the prefoamed bead filling operation, the chambers function as outlet spaces for air entering the mold cavity together with the prefoamed beads; during heating, foaming, and fusing thereof, they function as chambers for supplying steam to the mold cavity; and during cooling, they function as cooling compartments wherein cooling water may be sprayed onto the back faces of the core mold and cavity mold.
The method for molding a molded article using this type of molding apparatus is basically composed of the following four steps.
During the initial filling process, a pressure differential is created between the mold cavity and a starting material tank that contains prefoamed beads, and the prefoamed beads are carried from the starting material tank into the mold cavity on a stream of air so as to fill the mold cavity. Typical filling methods include cracked filling, compression filling, pressurized filling, and the like.
In a subsequent heating/fusing step, steam pressure is caused to act on the chambers, whereupon the prefoamed beads are heated and fused by steam entering the mold cavity via the air orifices. Since air remaining in the spaces between prefoamed beads can make it difficult for the prefoamed beads to fuse, steam is passed through the mold cavity to replace any air remaining in the spaces between prefoamed beads with steam before proceeding with heating/fusing.
In a subsequent cooling step, cooling water is directed onto the backs of the molds in order to cool the molded article. During this process the molded article is cooled indirectly via the molds by cooling water directed onto the molds from the back, and is also cooled directly by cooling water that penetrates into the mold cavity via the air orifices.
In a subsequent mold release step, the molds are parted and the molded article is released. The timing of mold parting is typically set with reference to time elapsed since the cooling water spray commences. When just released from the mold, the molded article has adequate shape retention due to the high vapor pressure and air pressure inside the beads, but with time the steam condenses and the molded article tends to contract. Accordingly, it is desirable that the molded article be adequately cooled, and where there are strict requirements as to the shape and dimensions of a molded article, the molded article may be set in a fixture until shape and dimensions stabilize.
The foam molding method described above is the principal molding method used currently. However, this molding method has a number of drawbacks, such as the following.
(1) To compensate for lower mold strength resulting from the molds being perforated by a multiplicity of air orifices, mold wall thickness in molds consisting of aluminum alloys must be of the order of 8-12 mm, for example. However, this has the effect of increasing the heat capacity of the mold, lowering thermal efficiency during heating/cooling so that the rate of temperature rise or temperature drop is slower, resulting in lower precision of control.
(2) Typically, a pair of molds is provided with some 2000 to 4000 air orifices, so the process of making the orifices is complicated and results in higher fabrication costs. Since the core vents are installed by hand in mounting orifices provided in the mold, the operation is quite complicated and damage to mold surfaces is unavoidable, thus requiring a retouching operation.
(3) Since clogging of air orifices such as core vents or core vent holes by scale or the like can result in heating defects, mold release defects, or cooling defects, the core vents must be replaced or periodically subjected to cleaning with high pressure water or to some other maintenance procedure.
(4) Since air orifices leave marks on foam molding surfaces, the visual appeal of molded articles suffers, and when the exterior surface is subjected to printing or the like, air orifice marks are an obstacle to attractive printing.
(5) Since the foam molded article is cooled by spraying cooling water into the chamber after molding, water infiltrates into the molding cavity through the air orifices, resulting in water content of about 6-10% in the molded article, necessitating a drying process. Further, since cooling water comes into direct contact with the molded article, cooling water quality must be controlled in order to produce uncontaminated molded articles.
(6) As all of the prefoamed beads are heated, expanded and fused under the same heating conditions by steam passing from the chamber into the mold cavity, molded articles produced in this way (hereinbelow referred to as isothermal molded articles) develop different surface qualities depending upon the extent of fusion of the beads. Specifically, lower fusion rates are associated with poor surface qualities, whereas higher fusion rates are associated with good surface qualities. For isothermal molded articles, higher bead fusion rates improve physical properties such as the mechanical strength of the molded article, but require longer heating, expansion/fusion times and cooling times, creating the problem of longer molding cycle times overall and reduced throughput.
For such reasons, in the molding technique described earlier, bead fusion rates in molded articles are typically set to 40%-80%, for example, in order to give good surface qualities and assure attractive appearance as well as assuring a fusion rate adequate to assure mechanical strength. However, even where mechanical strength requirements for a molded article are not particularly stringent, the need to assure an attractive appearance requires a moderately high fusion rate, which will result in a correspondingly longer molding cycle time and reduced throughput. Fusion rate as used herein is ascertained by splitting the molded article and observing the condition of the beads on the sectional face, specifically, by measuring the proportion of beads experiencing breakdown of the bead per se, deeming beads having cracking along the bead surface but no bursting of the bead per se to be unfused and deeming beads experiencing bursting of the bead per se into fragments to be fused.
The foam molding process described hereinabove is designed such that air orifices such as core vents and core vent holes are used to deliver steam, air, or other service fluids to the mold cavity or to evacuate same from the mold cavity during production of foam molded articles. However, as noted, the provision of air orifices creates a number of problems.
With the goal of providing a fundamental solution to these problems, the inventors conducted extensive research concerning development of a foam molding process which would employ molds devoid of air orifices, and conducted tests of various kinds. While the goal is a mold xe2x80x9cdevoid of air orifices,xe2x80x9d it is of course necessary to provide, in lieu of core vents and core vent holes, some kind of passages for delivering/evacuating steam, air, or other service fluids to and from the mold cavity, which gives rise to the issue of where and how to form same, of the timing and conditions that should be employed in delivering service fluids to such passages, and a host of other issues that need to be addressed.
One such problem is that of a decline in the efficiency of cooling of the molded article. When a mold is devoid of air orifices, cooling water sprayed onto the back face of the mold does not penetrate into the mold cavity, so the molded article is not cooled directly by cooling water but is rather cooled only indirectly via the mold. Thus, cooling efficiency will be lower than with a conventional molding apparatus that produces both direct and indirect cooling, and the time required to cool the molded article will be longer. Another problem, albeit one that is encountered with in-mold foam molding apparatuses whose molds do have air orifices as well, is that if the molded article is released from the mold before adequately cooling, the molded article per se will be soft and lack shape sustaining power, resulting a molded article having poor shape stability and experiencing problems such as deformation. Accordingly, it was considered to be difficult to reducing the molded article cooling time any further.
It is an object of the present invention to make practical a novel foam molding apparatus wherein the mold surfaces are devoid of air orifices such as core vents and core vent holes, and to provide an in-mold foam molding apparatus and in-mold foam molding method for polyolefin synthetic resins which addresses one of the issues of this novel foam molding technique, namely, reduced cooling time for molded articles.
As a result of exhaustive research concerning ways to reduce cooling time for molded articles, the Applicant noted that even if a molded article is released from the mold before it has adequately cooled and is still soft, the shape of the molded article can nevertheless be stabilized by setting it in a fixture, and arrived at the idea of dramatically shortening cooling time by releasing the molded article from the mold while still soft, but not so soft as to break. The Applicant further arrived at the idea that the timing for mold release can be set accurately and dimensional stability can be greatly improved by conducting a series of measurements of prefoamed bead bearing pressure against the molds and setting the timing of mold release on the basis of the measurements so obtained, and perfected the invention on the basis thereof.
The polyolefin synthetic resin in-mold foam molding method of the first phase of the present invention comprises the steps of filling a mold cavity with prefoamed beads comprising a polyolefin synthetic resin; heating and fusing these with steam; and cooling the molded article; wherein during the cooling process the pressure of the foamed resin against the molds is measured successively by means of a bearing pressure sensor, and when the pressure of the foamed resin against the molds is found to have reached a pressure predetermined for the particular molded article, cooling of the molded article is terminated, the molded article is released from the mold, and the molded article is then set in a fixture to stabilize the shape thereof.
According to this molding method, the released molded article is set in a fixture and held therein for a given time interval, whereby it is possible to stabilize the shape and dimensions of the molded article outside of the mold, thereby assuring adequate dimensional precision of the molded article, while at the same time shortening cooling time by releasing the molded article from the mold while still soft, but not soft enough to break, whereby molding throughput may be improved. As the timing for mold release is determined on the basis of serial measurement of the pressure of the foamed resin against the molds, mold release can timed optimally. In the in-mold foam molding apparatuses commonly used at present, the time elapsed from the outset of cooling is measured using a timer, and once a predetermined time interval has elapsed, cooling is terminated and the molded article is released. However, with such an arrangement mold release timing cannot be adjusted in response to variation in optimal mold release timing due reduced cooling efficiency resulting from variation in prefoamed bead diameter between shots, clogging of core vents, and the like. Accordingly, a rather long cooling time must be provided. According to the present invention, however, the timing for mold release is determined on the basis of foamed resin pressure, making it possible to time mold release in an optimal manner so as to appreciably reduce cooling time and improve dimensional stability of molded articles by controlling variation in molding dimensions between shots.
The molding method of the second phase of the present invention involves terminating cooling of the molded article and releasing the molded article when foamed resin pressure against the molds is within the range 0.02-0.2 MPa. Where foamed resin pressure is less than 0.02 MPa, no appreciable reduction in cooling time is achieved. Above 0.2 MPa the molded article is still too soft and the molded article may split when the molded article is released through ejection by an ejector pin, or the foaming pressure of the prefoamed beads will be so high so that the molded article expands when released, resulting in lower dimensional accuracy of the molded article. Accordingly, resin pressure is preferably set within the range 0.02-0.2 MPa.
The molding method of the third phase of the present invention involves setting the time for which the molded article is set in the fixture to between 5 and 60 minutes. While setting time will differ with prefoamed bead material and molding weight, a relatively large molded article may be imparted stable shape with setting for 15 to 40 minutes.
The molding method of the fourth phase of the present invention involves providing a plurality of fixtures which are cyclically delivered to the molding apparatus by means of a conveyor. With this arrangement, molded articles molded by the molding apparatus can be successively set in the fixtures delivered to the molding apparatus by the conveyor so that the molding process and the molding shape stabilization process may be carried out as a continuous operation.
The molding method of the fifth phase of the present invention employs as molds a core mold and a cavity mold that are devoid of air orifices such as core vents and core vent holes in mold sections that form conspicuous portions on the exterior of a molded article.
With this molded article method, mold sections for moldING conspicuous portions on the exterior of molded articles are devoid of air orifices, whereby marks left on molding surfaces by air orifices will be situated in inconspicuous locations on molding surfaces where they do not detract from the attractiveness of the surface of the molded article.
Further, with this molding method, air orifices can be completely or largely dispensed with, whereby it becomes possible to separately control delivery of service fluids to the chamber to the back of the core mold, to the chamber to the back of the cavity mold, and to the mold cavity. By controlling a service fluid, such as steam for example, so as to independently manipulate heating conditions in each compartment, it is possible by means of steam delivered to the two chambers to regulate the surface properties of the prefoamed beads filling the mold cavity in areas thereof contacting the core mold and the cavity mold, while at the same time heating, foaming, and fusing the prefoamed beads in the mold cavity by means of steam delivered to the mold cavity, whereby the rate of fusion of the prefoamed beads may be controlled independently of surface qualities. Thus, the rate of fusion in the interior of a molded article may be held to a low level, reducing molding cycle time while producing a molded article having an attractive surface, thus achieving both good throughput and high product value.
The molding method of the sixth phase of the present invention dispenses completely with air orifices in the two molds. With this arrangement, heating conditions in the three compartments, namely, the chamber to the rear of the core mold, the chamber to the rear of the cavity mold, and the core mold, can be precisely controlled, and the surface of the resultant molded article is free from marks produced by air orifices. The absence of air orifices also prevents cooling water from coming into direct contact with the molded article, thereby allowing the water content of the molded article to be held to a low level, obviating the need for a drying process after mold release, affording uncontaminated molded articles without the need to control cooling water quality, and providing other advantages.
In the molding method of the seventh phase of the present invention, during the process of cooling the molded article, cooling water is sprayed onto the back faces of the two molds by first cooling means in order to cool the molded article indirectly via the molds, and cooling water from outside the mold is sprayed by second cooling means into the mold cavity from cooling water orifices provided to at least one of the molds, whereby the molded article is cooled directly with cooling water.
As noted, eliminating air orifices from the molds means that during the cooling process the molded article is not cooled directly by cooling water, and cooling efficiency drops. According to this invention, however, the molded article can be cooled indirectly via the molds by spraying cooling water onto the back faces of the two molds using first cooling means, while also cooling the molded article directly with cooling water by spraying cooling water into the mold cavity from cooling water orifices provided to at least one of the molds, using second cooling means. Thus, despite the absence of air orifices in the molds, the molded article can be cooled efficiently and the time required to cool the molded article can be appreciably reduced, as well as reducing the amount of cooling water used. Since only the cooling water from the second cooling means comes into contact with the molded article, where it is desired to keep molded articles in an uncontaminated state, it is sufficient to implement control of cooling water quality for the cooling water of the second cooling means only, thereby significantly reducing the costs associated with water treatment.
The molding method of the eighth phase of the present invention involves providing cooling water orifices in proximity to the prefoamed bead filling unit, in proximity to the ejector pin, or both. Like air orifices such as core vents and core vent holes, cooling water orifices in the core mold and/or cavity mold leave marks on the molding surface, so it is preferable for these marks to be situated at inconspicuous locations on the molded article, such as in proximity to the prefoamed bead filling unit, in proximity to the ejector pin, or both.
In the molding method of the ninth phase of the present invention, cooling water is sprayed into the mold cavity through cooling water orifices by the second cooling means some 2 to 30 seconds after the first cooling means commences cooling by spraying cooling water.
When the prefoamed beads filling the mold cavity are subjected to vapor pressure, heated, and the mold cavity is then returned to atmospheric pressure, they expand and fuse together within in mold cavity with no gaps therebetween. If cooling water should be delivered to the mold cavity at this point, the prefoamed beads will harden before expanding adequately, resulting in a molding defect whereby height variation is produced on the molding surface. Since cooling of molded articles requires some time, commencing the cooling process by spraying cooling water on the back faces of the molds only after the prefoamed beads have expanded sufficiently will result in extended cooling time. Accordingly, cooling of the molded article by the first cooling means commences promptly upon completion of the heating process, and 2 to 30 seconds later, once the prefoamed beads have sufficiently expanded, cooling water is delivered to the mold cavity by the second cooling means in order to cool the molded article. This reduces the cooling time dramatically while giving molded articles of good quality.
The molding method of the tenth phase of the present invention involves successively measuring foamed resin pressure against the molds, and when the foamed resin pressure observed subsequent to the start of cooling by the first cooling means reaches a level 0.50 to 0.95 times the foamed resin pressure observed at the conclusion of heating, cooling water is sprayed into the mold cavity by the second cooling means. In the ninth phase of the present invention, the timing for operation of the second cooling means is delayed for some time after the first cooling means has begun to operate. This may be accomplished by making direct measurements of foamed resin pressure in the mold cavity, and when the foamed resin pressure observed subsequent to commencing spraying by the first cooling means reaches a level 0.50 to 0.95 times the foamed resin pressure at observed conclusion of heating, deeming the prefoamed beads to be sufficiently foamed and operating the second cooling means.
In preferred practice, operating time for the second cooling means will be from 2 to 30 seconds, as the eleventh phase of the present invention, and will be equivalent to 3 to 50% of operating time for the first cooling means, as the twelfth phase of the present invention. That is, while it is acceptable to continue to operate the second means until the end of the cooling process, since the cooling water delivered by the second cooling means comes into direct contact with the molded article, it is preferable to set the operating time as taught in the eleventh or twelfth phase of the present invention in order to assure adequate cooling efficiency while preventing shrinkage of the molded article due to over-cooling, to hold down the water content of the molded article, and to prevent contamination of the molded article after mold release.
The in-mold foam molded articles of the thirteenth phase of the present invention are molded into the shape of core for a car bumper by the molding method according to any of the first phase to twelfth phase of the present invention.
Car bumper cores of this kind must have highly accurate dimensions despite their considerable length, and are considered difficult to produce even with the extended cooling times employed in conventional methods. Through the use of the molding method described hereinabove, however, molding may be carried out without any drop in throughput.